The present invention relates to a process for the production of hollow monolithic elements of composite material, in particular carbon fibre formed in layers or sheets by impregnation of the fibres with thermosetting synthetic plastics resin, preferably epoxy resin. The present process is described with particular reference to the production of a hollow fork for bicycles, but can be applied to any form of hollow mechanical component.
Hollow mechanical elements made of composite material, in particular carbon fibre impregnated with epoxy resin, are known.
Such elements are normally formed as two half-shells which are then fixed together by adhesive and/or by other methods. The half-shells are obtained by means of a compaction of a plurality of sheets or mats of carbon fibre against a mould cavity by wrapping the mould in a bag within which a vacuum is formed and, subsequently, introducing the mould, bag and mats or sheets of carbon fibre into an autoclave where these latter are welded to one another by polymerisation and cured or "matured", in such a way as to obtain half-shells with the desired characteristics of mechanical strength, density and finish, then adopting a standard cycle, known to anyone skilled in the art, of heating and pressurisation (for example by maintaining the mould in an autoclave at three bar and 140.degree.-200.degree. C. for a time varying from several minutes to several hours, depending on the dimensions of the workpiece to be formed and the materials used.
However, hollow articles obtained by fixing together half-shells on the one hand can present inferior mechanical characteristics to those obtainable from a monolithic article of the same material and dimensions and, on the other hand, present an unsatisfactory aesthetic appearance in that the junction line remains visible; moreover, producing hollow articles by the connection of half-shells is relatively laborious, time consuming and expensive in that it requires an extra operation (that of connection) with respect to the production of a monolithic article, as well as the precise positioning of the components to be joined. On the other hand, with the known techniques it is substantially impossible to produce monolithic hollow workpieces of all shapes in composite material in that, in order to produce them, it is evidently necessary to utilise an entirely round core which must be rigid in order to permit compaction of the sheets or mats before polymerisation. Therefore, except in the case of cylindrical or conical articles, that is of simple form, it is currently impossible to eliminate the core from the interior of the finished article, with a consequent considerable increase in the weight of the finished article; this disadvantage remains even when utilising cores of gypsum, in that these cannot be completely eliminated, not even by the introduction of water or steam. On the other hand, the use of a "lost" core which can easily be eliminated is currently impossible in that, on the one hand, the polymerisation temperatures are well above the fusion temperature (or softening temperature) of wax or thermoplastic cores and, on the other hand, cores of expanded plastics materials which can be eliminated using solvents are not rigid so that they would squash if subjected to the pressures commonly utilised in the polymerisation stage.